Thermoplastic vulcanizates include blends of dynamically cured rubber and thermoplastic polymers. The rubber may be dispersed within the thermoplastic resin phase as finely-divided rubber particles. These compositions have advantageously demonstrated many of the properties of thermoset elastomers, yet they are processable as thermoplastics.
Because thermoplastic vulcanizates are advantageously processable as thermoplastics, elastomeric articles are often extruded from thermoplastic vulcanizates using thermoplastic extruding techniques. In certain situations, it is desirable for the extruded article to have a surface appearance characterized by surface smoothness. The ability to achieve high quality smoothness has long been a challenge in the thermoplastic vulcanizate art.
For example, U.S. Pat. No. 6,451,915 teaches thermoplastic vulcanizates including linear polyolefin resins or diene-modified polyolefin resins that have a melt flow rate that is greater than 1,000 dg/min and a viscosity average branching index that is from about 0.4 to about 0.95. These thermoplastic vulcanizates advantageously demonstrate an improved processability and surface appearance without deleteriously sacrificing mechanical properties.
U.S. Patent Application Publication No. 2006/0293454 describes a process for preparing low durometer thermoplastic vulcanizate compositions that includes introducing process oil into the melt blending before, during, and/or after the addition of a cure agent to the extruder. When a preferred combination of kneading blocks was employed, improved surface appearance of extruded profiles could be achieved.
U.S. Patent Application Publication No. 2007/0135571 teaches a process for the preparation of thermoplastic vulcanizates where a process oil is added to the extruder in at least three oil injection positions. This technique yields thermoplastic vulcanizates capable of being extrusion molded with a smooth surface and low surface spot count in addition to excellent elastomeric properties.